How do you check if an object has a specific property?

TL;DR

To check if an object has a specific property, you can use the in operator or the hasOwnProperty method. The in operator checks for both own and inherited properties, while hasOwnProperty checks only for own properties.

const obj = { key: 'value' };

// Using the `in` operator
if ('key' in obj) {
  console.log('Property exists');
}

// Using `hasOwnProperty`
if (obj.hasOwnProperty('key')) {
  console.log('Property exists');
}

How do you check if an object has a specific property?

Using the in operator

The in operator checks if a property exists in an object, including properties in the object's prototype chain.

const obj = { key: 'value' };

if ('key' in obj) {
  console.log('Property exists');
}

Using hasOwnProperty

The hasOwnProperty method checks if a property exists directly on the object, not in its prototype chain.

const obj = { key: 'value' };

if (obj.hasOwnProperty('key')) {
  console.log('Property exists');
}

Differences between in and hasOwnProperty

• The in operator checks for both own and inherited properties.
• The hasOwnProperty method checks only for own properties.

Example with inherited properties

const parentObj = { inheritedKey: 'inheritedValue' };
const childObj = Object.create(parentObj);
childObj.ownKey = 'ownValue';

console.log('inheritedKey' in childObj); // true
console.log(childObj.hasOwnProperty('inheritedKey')); // false
console.log('ownKey' in childObj); // true
console.log(childObj.hasOwnProperty('ownKey')); // true

Further reading

• MDN Web Docs: in operator
• MDN Web Docs: Object.prototype.hasOwnProperty